1. Field of the Invention
The present invention relates to an apparatus for continuously electrowinning copper from a slurry comprising copper-bearing solids and an electrolyte.
2. State of the Art
Due to environmental problems inherent in conventional copper recovery techniques such as smelting, utilization of electrowinning techniques for recovering copper has been receiving increased attention.
The majority of electrowinning techniques recover copper from a clear, copper-bearing electrolyte. That is, copper-bearing solids are dissolved in an electrolyte and the resulting solution is electrolyzed in a tank having anode and cathode electrodes which are immersed in the solution. Positively charged copper ions in the electrolyte solution migrate to the cathode and deposit upon the cathode as elemental copper when an electric current is passed through the solution. For a typical sulfate-based electrolyte solution, copper is deposited at the cathode and oxygen gas is evolved at the anode.
One type of copper electrowinning device utilizes a diaphragm to divide the device into separate anode and cathode compartments. Such diaphragm devices are utilized when the electrolyte contains an oxidizable component which is oxidized at the anode. The oxidizable component is retained in an anolyte within the anode compartment to isolate it from the cathode where it could, in turn, be reduced. When the oxidizable component is oxidized in the anolyte of a diaphragm cell, it is normally used subsequently to oxidize copper-bearing feed material to replenish copper in solution for further electrowinning.
In some cases, a solid copper-bearing feed material is mixed with electrolyte and the slurry is fed directly to the anode compartment of the diaphragm cell. In this case, oxidation of the oxidizable electrolyte component and of the copper mineral takes place simultaneously.
A typical slurry electrowinning device includes a tank which contains a number of alternating, spaced-apart cathode and anode electrodes. Crushed copper-bearing feed material is slurried with a suitable electrolyte and fed into the tank. A maximum electrolyte level is maintained in the tank by an overflow outlet. The overflowing electrolyte is continuously recycled to the feed inlet of the tank to be slurried with further crushed feed material. Either mechanical mixers or gas spargers are utilized to agitate the slurry to ensure uniform circulation of the slurry across the faces of the cathode and anode electrodes. Alternatively, the electrodes themselves are oscillated to provide the proper agitation. Current is passed through the electrolyte with the result that copper deposits on the cathodes. Periodically, the cathodes are removed from the tank and the deposited copper is harvested.
Conventional slurry electrowinning devices are typically plagued by a number of problems. First, the feed inlet to the electrolysis tank is often above the solution level in the tank. The fall of the feed solution into the tank entrains air and causes a heavy froth to form at the top of the tank. This froth often overflows from the top of the tank, introducing a highly acidic liquid to the nearby work environment and, at the same time, depleting the electrolyte solution of valuable solids. Second, attempts to eliminate the above problem by placing a hood or cover over the tank have proven impractical in commercial operations. To remove copper-laden cathodes from the covered tank, it is necessary to stop circulation in the tank, disconnect the feed piping and remove the cover which is dripping with acidic solution. With the tank's circulation system shut down, solids settle to the bottom of the tank, compact and plug the unit. Third, past efforts to eliminate the above problems by utilizing a feed inlet in the side of the tank have been largely unsuccessful because the side feed has resulted in uneven flow distribution within the tank, the flow entering the tank at high velocity and at right angles to the desired flow between the anodes and cathodes.